1. Technical Field
This invention relates to a small size electric cable primarily for telephone, data and other signal transmissions where cable tensile strength, flexibility, and flat cable ductility are the major concerns.
2. Background Art
In the electronics field there is a general class of flexible cables known as tinsel cables. Tinsel cables are used in applications where great flexibility for the cable is required. Generally they are constructed by spiral wrapping a tensile foil of conductive material, usually copper or copper alloy, around a tensile filament or element, usually nylon or polyester. The wire is then coated with a thermoplastic insulating material. The required number of independent wires are then arranged in a ribbon and jacketed with a second plastic material to form a multi-wire, flexible cable, which can be subjected to repeated flexure without fatiguing the conductive tensile metal foil.
In the past, the primary structural member able to withstand tensile stress in these prior art flexible cables was the plastic jacket. However, there was present a trade-off between tensile strength of the cable and ductility. In order to assure a flexible cable having high tensile strength, the cross-sectional area of the plastic jacket was increased, which resulted in a decrease in ductility. Conversely, as the cables were miniaturized by minimizing the cross-sectional area of the plastic jacket, ductility increased but tensile strength decreased. At the dimensional sizes taught by the present invention, there is insufficient plastic material in the plastic jacket to be of any significant use as a structural member able to withstand even moderate tensile stress.
To compensate for the loss of tensile strength resulting from miniaturization or reduction in the cross-sectional area of the plastic jacket, aramid fibers from the family of aromatic polyamides were substituted for the nylon and polyester filaments used in the past. These aromatic polyamides have, in addition to high tensile strength, another favorable property over the older nylon and polyester filaments, namely they are relatively inelastic. Nylon and polyester tensile filaments are subject to elongation factors of ten per-cent at strain forces of a mere 4 grams/denier (35 cN/Tex) and will break at force levels of approximately 8 grams per denier (70 cN/Tex). These forces can be easily incurred in miniature cables by inadvertently tugging on the cable or, in a localized fashion, merely by folding and crimping the cable.
The elasticity of the nylon and polyester filaments cause problems with single wraps of tinsel when wrapped in a helical spiral fashion about each filament, in that the elasticity of the filament greatly exceeded that of the copper or the copper alloy tinsel foil. This resulted in a loss of, or reduced, conductivity and eventual breakage of the cable.
To compensate for this, it is standard practice in the industry to provide for two wraps of tinsel foil about each cable. To insure electrical conductivity, each of these wraps is, as taught in the prior art, wrapped in a helically spiral opposite to the other, that is to say, one in a clockwise direction, and the other in a counter-clockwise direction to solve the problem of maintaining good conductivity under conditions of tensile stretching in cables having nylon or polyester tensile filaments.
The opposing spiral design, originally adopted to compensate for tensile stretching, has been carried over into the new non-elastic tensile filament cables using aromatic fibers. But this design has an inherent defect, in that if the cable is twisted, it will wrap the helical spiral of tinsel tighter in one direction, and unwrap the tinsel foil which was wrapped in the other direction. This results in an abrasion of metal-to-metal rubbing between the two helical spiral wraps. In practice it has been found that there is a significant amount of abrasion between the opposing spiral wraps, and eventual cutting of the outer wrap into the inner wrap and a resulting loss of conductivity or cable failure.
In practice it has been found that if both wraps of tinsel foil are made in the same direction, there is less abrasion, better conductivity, and an extended cable life. However, unidirectional double wrapping is not done because it induces a torsional stress into the conductive wire in the opposite direction from that in which the coils are wrapped, by reason of the coils tending to unwrap themselves from the filament. In cases of extremely ductile miniature cables, this actually can result in a multi wire cable assuming a helical spiral in a direction opposite that to which the tinsel is wrapped inside the cable.
Accordingly, it is an object of this invention to provide a miniature electric cable of high tensile strength, small cross-sectional area, with maximum wear unidirectional multiple layered spirals of conductive tinsel foil that will lie flat even though it is extremely ductile.